The invention relates to a method for evaluating two scattered-light signals in an optical hazard alarm operating in accordance with the scattered light principle.
The invention further relates to an optical hazard alarm with a detection unit operating in accordance with the scattered light principle.
Furthermore it is generally known that particles with a size of larger than 1 μm predominantly involve dust, while particles with a size of smaller than 1 μm predominantly involve smoke.
Such a method or such a hazard alarm is known from international publication WO 2008/064396 A1. In the publication it is proposed, for increasing the sensitivity for the detection of smoke particles, that only the second scattered light signal with blue light wavelength is evaluated if the amplitude ratio corresponds to a particle size of less than 1 μm. If on the other hand the amplitude ratio corresponds to a particle size of more than 1 μm, the difference is formed between the second scattered light signal with blue light wavelength and the first scattered light with infrared light wavelength. Through the differentiation the influence of dust is suppressed and thus the triggering of a false alarm for the presence of a fire is largely suppressed.
U.S. Pat. No. 7,738,098 B2 likewise discloses a method and also an optical hazard alarm for evaluation of two scattered light signals. The particles to be detected, present in a fluid are irradiated with light in a first wavelength range, such as e.g. in the blue wavelength range, and with light in a second wavelength range, such as in the red or infrared range for example. The two scattered light signals are subsequently normalized in relation to one another such that their amplitude curve roughly coincides for larger particles such as dust and steam, such as to Portland cement as a dust substitute for example.